Why do the dead not avail the living though Musa helped Muhammad?

Q 1: Some people wonder how we can claim that the dead avail nothing, although Musa (Moses, peace be upon him) after his death availed our Prophet Muhammad (peace be upon him) during the Isra' (Night Journey) and Mi`raj (Ascension to Heaven). He advised our Prophet to return to Allah and ask him to reduce the daily obligatory prayers from fifty to five. Also, how can we claim that the Messenger (peace be upon him), the best of creation, avails us nothing after his death?

A: The advice that Musa (peace be upon him) gave our Prophet (peace be upon him), that he should ask Allah for lightening the duty of prayers from fifty to five daily Prayers, (Part No. 1; Page No. 173) and what happened with the other prophets whose dead bodies exist in their graves on earth are a special case and one of the miraculous supernatural events restricted only to the Prophet. Allah is over all things competent. The prophets are dead and lead in their graves a life of Barzakh (period between death and the Resurrection). Allah relieved their bodies from decay. However, this case or any other does not indicate that the prophets or any other dead people can avail or harm living people.May Allah grant us success. May peace and blessings of Allah be upon our Prophet Muhammad, his family and Companions!